Stereoscopic three-dimensional metrology system and method

ABSTRACT

A stereoscopic three-dimensional optical metrology system and method accurately measure the location of physical features on a test article in a manner that is fast and robust to surface contour discontinuities. Disclosed embodiments may image a test article from two or more perspectives through a substantially transparent fiducial plate bearing a fiducial marking; camera viewing angles and apparent relative distances between a feature on a test article and one or more fiducials may enable accurate calculation of feature position.

This application is a continuation of, claims the benefit of priorityfrom and incorporates herein by reference U.S. non-provisionalapplication Ser. No. 10/323,720 entitled “STEREOSCOPIC THREE-DIMENSIONALMETROLOGY SYSTEM AND METHOD,” filed Dec. 18, 2002 and which claimedbenefit from U.S. provisional application Ser. No. 60/346,447 entitled“APPARATUS FOR STEREOSCOPIC THREE-DIMENSIONAL METROLOGY,” filed Dec. 28,2001.

FIELD OF THE INVENTION

Aspects of the present invention relate generally to metrology, and moreparticularly to a system and method of accurately measuring thethree-dimensional location of physical features on a test article in amanner that is fast and robust to surface contour discontinuities.

DESCRIPTION OF THE RELATED ART

Several three-dimensional (3D) optical metrology techniques aregenerally known and currently practiced. These methods includestructured lighting, moire and laser interferometry, laser rangefinding, and a conventional two camera approach. Each of theconventional methods suffers from significant deficiencies at least withrespect to measurement applications: requiring high speed andparallelized information acquisition; in which surface contours mayinclude steep sloping surfaces or surface discontinuities; and in whichit is desirable to avoid expensive, high accuracy motion stages. Thestructured lighting and interferometric methods, for example, providepoor results with respect to measuring features on a test article havingsteeply sloping surfaces or surface discontinuities. The laser rangefinding method is slow and generally inaccurate. The traditional twocamera approach relies upon stage accuracy.

Laser interferometry systems direct a focused laser beam onto a testarticle and interfere the sensed reflected beam with a reference beam.Fringe pattern shifts are counted to infer variations in distance fromthe laser source. These systems and methods can be highly precise, butare generally limited in at least the following respects. Continuitybetween images is essential in order that such systems may maintain anaccurate reference. Accordingly, laser interferometric techniques do notpermit omission of uninteresting regions of the test article; this slowsperformance. Further, steeply pitched surfaces or surfacediscontinuities (e.g., sharp vertical edges or one object disposed ontop of another) can result in erroneous data.

Moire interferometry systems and methods project a reference grid from aparticular direction onto a test article while viewing the test articlefrom another direction; distances can be inferred from observedvariations in spacing between grid lines caused by sloping surfaces.This is a relatively fast imaging strategy that can capture an entiredisplacement field from a single measurement. Similar to laserinterferometry, however, steeply pitched surfaces and surfacediscontinuities can result in erroneous data.

Laser range finding methodologies direct a focused laser beam onto atest article and employ time-of-flight measurements to compute distance.The beam must be tightly focused when measuring distances to small,densely packed objects such as features on a semiconductor wafer, forexample. Accordingly, laser range finding is a relatively slow process,as it is generally limited to capturing only a single measurement foreach beam location. Further, this method does not provide sufficientresolution to ensure accurate measurements at the sub-micron level.

Structured lighting methods project precise bands of light onto a testarticle. Deviations in light band line straightness, when viewedobliquely, are translated into depth information. This method suffersfrom the following limitations: similar to the interferometrictechniques, steeply pitched surfaces and surface discontinuities canresult in erroneous measurement data; and specular reflections from thehighly focused bands of light can also lead to erroneous data.

The traditional two camera system acquires images using a first camerapositioned, for example, directly above the test article to obtainlateral measurements (i.e., x and y coordinates); a second camera,imaging from a different perspective or optical axis, is used totriangulate feature height (i.e., z dimension). This approach isgenerally limited by the precision with which the stage supporting thetest article can be moved. Stage inaccuracies translate directly intomeasurement error; accordingly, extremely precise stage motion isrequired for this approach, particularly in cases where the features onthe test article are small and closely packed.

SUMMARY

Embodiments of the present invention overcome the above-mentioned andvarious other shortcomings of conventional technology, providing astereoscopic three-dimensional optical metrology system and method ofaccurately measuring the location of physical features on a test articlein a manner that is fast and robust to surface contour discontinuities.As set forth in detail below, a system and method of three-dimensionaloptical metrology may image a test article from two or more perspectivesthrough a substantially transparent fiducial plate bearing a fiducialmarking. Camera viewing angles and apparent relative distances between afeature on a test article and one or more fiducials may enable accuratecalculation of feature position.

In accordance with one embodiment, for example, a method of measuring alocation of a physical feature on a test article comprises: supporting atest article to be imaged; interposing a fiducial plate bearing afiducial between the test article and an imaging device; imaging afeature of the test article and the fiducial; and measuring a locationof the feature relative to the fiducial.

The foregoing supporting may comprise utilizing a stage movable alongany of three coordinate axes; additionally or alternatively, the stagemay be rotatable about any of the three axes. As set forth in detailbelow, imaging may comprise selectively orienting the stage relative tothe imaging device, or selectively orienting the imaging device relativeto the stage.

In particular, imaging may comprise selectively translating an imageplane of the imaging device relative to the test article; in thatregard, selectively translating an image plane may comprise moving theimaging device relative to the test article or moving the test articlerelative to the imaging device.

In some embodiments, the imaging may comprise acquiring first image datafrom a first perspective and second image data from a secondperspective; it will be appreciated that the acquiring may compriseobtaining the first image data on a first image plane oriented at afirst angle relative to the article and obtaining the second image dataon a second image plane oriented at a second angle relative to thearticle. The first angle and the second angle may be equal.

In accordance with one exemplary implementation of the foregoingembodiment, the acquiring comprises: obtaining the first image data whenthe imaging device is at a first location relative to the article;selectively adjusting the relative positions of the imaging device andthe article; and obtaining the second image data when the imaging deviceis at a second location relative to the article.

In accordance with some exemplary methods, measuring a location of thefeature relative to the fiducial comprises computing an apparentdistance between the feature and the fiducial. Selectively repeating theimaging and the measuring may improve the accuracy of a positionalmeasurement.

In one embodiment, for example, the fiducial plate bears a plurality offiducials and the measuring comprises computing apparent distancesbetween the feature and selected ones of the plurality of fiducials.

In accordance with one disclosed embodiment, a metrology systemcomprises: a stage operative to support an article to be imaged; animaging device selectively oriented relative to the stage and operativeto acquire image data on an image plane; and a fiducial plate, bearing afiducial, interposed between the article to be imaged and the imagingdevice. The imaging device may be operative to image the fiducial and afeature on the article.

The system may further comprise an image processing component operativeto compute an apparent distance between the feature and the fiducialfrom the image data.

At least one of the stage and the imaging device may be movable alongany of three coordinate axes and may additionally or alternatively berotatable about any of the three axes. In some such embodiments, thesystem may further comprise a control element operative selectively tocontrol relative movement of the stage and the imaging device.

In some exemplary embodiments, the imaging device comprises acharge-coupled device image sensor, a complementary metal oxidesemiconductor image sensor, or a similar image sensor device.

The system may be implemented wherein the fiducial plate bears aplurality of fiducial markings. In such a system, the image processingcomponent may be operative to compute apparent distances between thefeature and selected ones of the plurality of fiducial markings.

In accordance with some embodiments, the imaging device acquires firstimage data from a first perspective relative to the article and secondimage data from a second perspective relative to the article. The firstimage data may be obtained when the image plane is oriented at a firstangle relative to the article and the second image data may be obtainedwhen the image plane is oriented at a second angle relative to thearticle. As noted above, the first angle and the second angle are equalin some implementations.

In one exemplary arrangement of the system: the first image data areacquired when the imaging device is at a first location relative to thearticle; the relative positions of the imaging device and the articleare selectively adjusted; and the second image data are acquired whenthe imaging device is at a second location relative to the article.

As set forth in detail below, the article may comprise a semiconductorwafer, chip, or die, for example.

The foregoing and other aspects of various embodiments of the presentinvention will be apparent through examination of the following detaileddescription thereof in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are simplified functional block diagrams illustratingone embodiment of a stereoscopic three-dimensional metrology system.

FIGS. 2A and 2B are simplified functional block diagrams illustratinganother embodiment of a stereoscopic three-dimensional metrology system.

FIG. 3 is simplified diagram illustrating geometric aspects of ametrology system constructed and operative in accordance with theembodiment of FIGS. 1A and 1B.

FIG. 4 is simplified diagram illustrating geometric aspects of ametrology system constructed and operative in accordance with theembodiment of FIGS. 2A and 2B.

FIG. 5 is a graph representing data for planarity and alignmentmeasurement sensitivities as functions of imaging device viewing angle.

FIG. 6 is a simplified diagrammatical side view of one embodiment of aprobe card analyzer system constructed and operative in accordance withthe embodiment of FIGS. 1A and 1B.

FIG. 7 is a simplified flow diagram illustrating the general operationof one embodiment of a stereoscopic three-dimensional metrology method.

FIG. 8 is a simplified flow diagram illustrating the general operationof one embodiment of an image data acquisition operation.

DETAILED DESCRIPTION

As set forth in detail below, aspects of the present system and methodmay minimize or eliminate the foregoing significant deficiencies ofconventional metrology approaches, specifically: (1) speed or throughputrate limitations; (2) measurement difficulties associated with surfacediscontinuities; and (3) reliance upon the accurate motion of stages.

The exemplary embodiments acquire field measurements, not pointmeasurements. In particular, each imaged field of view may produceposition measurements for every identified feature in a view, andaccordingly, a high degree of parallelism may be achieved. As aconsequence, position measurement throughput is significantly greaterthan can be attained using point measurement methods such as laser rangefinding. Further, unlike the interferometric strategies and thestructured light approach, a system and method operative in accordancewith the present disclosure do not require image location continuity toensure a tie to an established reference. Position measurements may betaken for any sampling of positions on the test article without the needto gather data on intermediate locations.

Interferometric methods yield position measurements relative to previousmeasurements. In order to measure the position of a particular point ona test article, therefore, interferometer systems must accumulatedistance variations from point to point, moving from a reference pointto the final point of interest. Any disturbance, anomaly, error, orother variation in this accumulation caused by surface discontinuitieswill produce errors in all downstream (i.e., subsequent) measurements.The system and method set forth herein, however, yield positionmeasurements relative to an established reference point, i.e., one ormore fiducial marks. Accumulation of distance variations is notnecessary. Aside from possible occlusion of areas of interest, steeplypitched or discontinuous surfaces do not prevent accurate measurements.

Further, a stereoscopic optical metrology system constructed andoperative in accordance with the present disclosure eliminates the needfor highly accurate stages; the burden of accuracy may be shifted to thefiducials and the imaging system. Manufacturing a fiducial plate togiven accuracy tolerances is significantly easier (and consequently,substantially less costly) than manufacturing a stage (and the attendantactuators, control mechanisms, and other hardware) to perform accuratelywithin those tolerances.

Although one particular application for the exemplary embodiments is themeasurement of semiconductor industry probe card planarity andalignment, those of skill in the art will readily appreciate that theutility of the present system and method is equally applicable in otherarenas.

Turning now to the drawing figures, it is noted that FIGS. 1A-B and 2A-Bare simplified functional block diagrams illustrating respectiveembodiments of a stereoscopic three-dimensional metrology system. Theunderlying principle of operation involves measuring the two Cartesiancomponents of distance between a feature of interest and a fiducial fromtwo different perspectives. Due to parallax, the distance between thefeature and the fiducial, when observed from each of the twoperspectives, will be measurably different. Given these two apparentdistances and the relative positions of the two different perspectiveviews, it is possible to calculate the true location, inthree-dimensional (3D) space, of a feature relative to a fiducial.

As illustrated in FIGS. 1A and 1B, one way to gather two perspectiveimages is to use two cameras or imaging devices 111 and 112 separated bysome distance; each imaging device may be configured and oriented toobserve and to image the same scene. In that regard, FIGS. 1A and 1Bdepict a fiducial object 199, a feature 129 (the location of which is tobe calculated), and two angular perspectives from which imaging devices111, 112 may view these objects.

As illustrated in FIGS. 2A and 2B, another approach to gathering twoperspective images is to use a single imaging device 110 that may scanalong a particular direction (e.g., the x direction in FIG. 2B) relativeto the article 120 to be imaged. Imaging device 110 may capture imagesof the same feature 129 and fiducial 199 at different points orlocations during the scan. The changing apparent distance betweenfeature 129 and fiducial 199 from one side of the field of view to theother may be used, in the same manner as the approach depicted in FIGS.1A and 1B, to calculate a position of feature 129.

In the exemplary embodiments, system 100 generally comprises an imagingdevice 310, or a pair of imaging devices 111, 112, operative to acquireimage data of a 3D object space, in general, and of a test article 120,in particular. An object or feature 129 of interest to be imaged may besupported or disposed on test article 120; in some embodiments, forexample, test article 120 may be embodied in or comprise a microscopeslide, a microarray, a microtiter or other multi-well plate, asemiconductor chip, die, or wafer, or any other similar structureconfigured and operative to support objects, specimens, sample material,features, and the like for viewing or imaging.

Test article 120 may be fixedly or movably attached to, or otherwisedisposed on, a precision motion stage 130 or other movable supportelement, and may be moved or translated through precise motion of stage130. In that regard, test article 120 may be movable in any or all ofthe x, y, and z directions, as is generally known in the art; thismovement may be accomplished through translation of test article 120itself (i.e., relative to stage 130), through motion of stage 130 or anyother apparatus upon which test article 120 is disposed, or both.Accordingly, selective translation of test article 120 along one or morecoordinate axes may allow article 120 and feature 129 to be selectivelypositioned at a suitable location (relative to imaging devices 110 or111, 112) for imaging. Additionally or alternatively, test article 120,stage 130, or both may be rotatable about one or more coordinate axes.

Numerous and varied apparatus and methods of providing controlledmovement or translation of test article 120 and stage 130 are known andwell within the capabilities of an ordinarily skilled artisan. The scopeof the present disclosure is not intended to be limited by anystructures and techniques employed to manipulate or to orient testarticle 120. As an alternative, for example, since the functionality andaccuracy of the exemplary embodiments are not dependent upon accuratemotion of stage 130 as set forth above, it will be appreciated thatstage 130 and test article 120 may be fixed relative to the x, y, and zaxes. In such alternative embodiments, any relative movement necessaryto provide different imaging perspectives may be effectuated throughselective movement of imaging devices 110 or 111, 112 relative to one ormore coordinate axes.

In some implementations, imaging devices 110-112 may be embodied in orcomprise a camera incorporating charge-coupled device (CCD) technology,for example, or complementary metal oxide semiconductor (CMOS) imagesensors. Additionally or alternatively, imaging devices 110-112 maycomprise supplementary optical elements or imaging components such aslens arrays or serial focusing elements, optical microscopes, scanningelectron microscopes (SEM), spectrophotometers, or any other apparatusor instrument configured and operative in conjunction with image sensorsor sensor arrays to acquire video or image data.

Imaging devices 110-112 may additionally comprise or be coupled to oneor more image processing components (such as image processor 161)operative to process, store, or otherwise to manipulate captured imagedata as desired. Such image processing components may comprise one ormore microprocessors or microcontrollers, for example, capable ofexecuting software code or other instruction sets for interpolating,extrapolating, filtering, deconvolving, or otherwise manipulating imagedata captured by and transmitted from devices 110-112. Image processor161 may execute or run a real-time operating system, for example,enabling reconfiguration or selective programming of processorfunctionality. In particular, it is noted that image processor 161,either independently or in conjunction with one or more additional imageprocessing components, may be employed to perform the computations andoperations set forth in detail below.

As is generally known in the art, some image processing techniques areprocessor intensive (i.e., computationally expensive) and requiresignificant computing power and other resources for data manipulationand storage. Accordingly, image processor 161 may additionally comprisecomputer readable storage media such as: read-only memory (ROM); randomaccess memory (RAM); hard or floppy disk drives; digital versatile disk(DVD) drives; or other magnetic, optical, or magneto-optical computerstorage media and attendant hardware. Sufficient storage media may beprovided to support the computational functionality described in detailbelow with reference to FIGS. 3-5, as well as to enable reconfigurationor selective programming as noted above.

In a manner similar to that of test article 120 and stage 130, imagingdevices 110-112 may be movable in any or all of the x, y, and zdirections; accordingly, selective movement or translation of devices110 and 111,112, or of one or more components thereof, along one or morecoordinate axes may enable precise positioning of a focal plane withrespect to test article 120. Various apparatus and methods of providingcontrolled movement of devices 110-112 or providing accurate placementof focal planes are generally known in the art. In that regard, devices110-112 may be operably coupled to guide rails or tracks, steppermotors, articulated arms, or other automated structures or roboticsystems operative selectively to position devices 110-112 relative totest article 120 for imaging operations.

Additionally, device 110 and in particular, devices 111 and 112, or oneor more components thereof, may be rotatable about one or more of the x,y, and z coordinate axes. In that regard, devices 110-112 may beoperably coupled to or mounted on appropriate hardware such as hinges,gimbals, journal and bearing assemblies, or other pivotable structurescapable of selectively orienting, supporting, and maintaining devices110-112 at a predetermined or dynamically adjustable angle relative toone or more coordinate axes. In some embodiments, selective or dynamicrotation of devices 110-112 about one or more axes may not be necessary;in such embodiments, devices 110-112 may be fixed at a particularangular orientation to support the functionality set forth below.

The scope of the present disclosure is not intended to be limited by anystructures and techniques employed to manipulate devices 110-112. Itwill be appreciated that relative motion between devices 110-112 andstage 130 in general, and test article 120 in particular, may beimplemented or controlled in numerous ways.

In that regard, system 100 may further comprise one or moremicroprocessors, microcontrollers, or other electronic devices (such ascontrol electronics 162) operative to control relative movement,positioning, and orientation of devices 110 or 111, 112 and test article120. As indicated in FIGS. 1B and 2B, control electronics 162 may beoperably coupled to image processor 161 described above or to otherimage processing components. In some embodiments, image processor 161may initiate, execute, or terminate scanning or image captureoperations, for example, responsive to control signals or other data(e.g., indicative of placement or relative movement of devices 110 or111,112 and test article 120) received from control electronics 162.Similarly, control electronics 162 may receive data or instructions sets(e.g., relating to desired movements or the timing thereof) from imageprocessor 161, and may arrange or orient devices 110 or 111, 112 andtest article 120 accordingly. It will be appreciated that thefunctionality of image processor 161 and control electronics 162 may becombined, incorporated, or integrated into a single device or hardwarearrangement.

In the embodiments depicted in FIGS. 1A-B and 2A-B, system 100 furthercomprises a fiducial plate 190 disposed, supported, or otherwisemaintained between devices 110 or 111, 112 and test article 120. Plate190 may be operative to carry or to display one or more markings orreference indicia, such as fiducial 199, which may be arranged randomly,for example, or in an ordered array or predetermined orientation. Onlyone fiducial 199 is illustrated in the drawing figures for clarity; itis noted, however, that the illustrations are not intended to beinterpreted in any limiting sense.

Fiducial plate 190 may be constructed or fabricated to be substantiallytransparent, enabling imaging of article 120, feature 129, and fiducial199 while plate 190 is interposed between article 120 and imagingdevices 110 or 111, 112.

Such transparency may be facilitated or influenced by, for example, theproperties of the material or composition used to construct plate 190,the thickness, polish, planarity, and other surface characteristics ofplate 190, and so forth. In this context, therefore, the term“substantially transparent” generally refers to a fiducial plate 190constructed of suitable materials and having appropriate dimensionalcharacteristics such that plate 190 neither prohibits optical or otherimaging of article 120 therethrough nor otherwise interferes with thefunctionality and operation of imaging devices 110-112 as set forthherein.

In some implementations, plate 190 may be embodied in or comprise glass,quartz, acrylic, sapphire, silica, or any other material known to haveappropriate material properties to support the imaging functionality ofimaging devices 110-112 and system 100. Fiducial 199 or a fiducial arrayas described above may be etched onto a surface, supported or suspendedwithin, or otherwise incorporated into plate 190.

When interposed between imaging devices 110 or 111, 112 and test article120, plate 190 may be supported or otherwise fixed at a known locationor orientation relative to the 3D coordinate axis system (e.g., theorigin in FIGS. 1B and 2B, where x=0, y=0, z=0). Accordingly, thelocation of fiducial 199 or other reference indicia in 3D space may beaccurately computed and subsequently employed to calculate a locationfor feature 129 with respect to the coordinate axis system, in general,and with respect to other features or reference points on test article120, in particular.

In that regard, fiducial 199 may be so dimensioned or constructed toenable selective imaging thereof In some embodiments, for example,fiducial 199 may be configured or fabricated to be substantiallytransparent during operations employing a particular imaging modality.For instance, fiducial 199 (or each of a plurality of fiducials arrangedin an array) may be suitably sized or colored only to appear visible oropaque under certain conditions, rending fiducial 199 substantiallytransparent to incident light of a particular wavelength or frequencyband, for example, employed to image feature 129. In such embodiments,providing illumination of a different wavelength or otherwise switchingthe imaging modality may selectively enable imaging of fiducial 199.

It will be appreciated that both feature 129 and fiducial 199 may stillbe imaged in these implementations, though fiducial 199 may only beopaque in one of the two selected modalities. In accordance with theforegoing embodiment, feature 129 may be imaged from a particularperspective using a particular modality rendering fiducial 199transparent, the imaging technique may be altered to employ a secondmodality, and fiducial 199 may then be imaged from the same perspective(it is noted that feature 129 may also be imaged in the secondmodality). Image data acquired during the two imaging operations may becombined to provide an accurate representation of both fiducial 199 andfeature 129 in a single composite image frame. Accordingly, an apparentdistance between fiducial 199 and feature 129 may be obtained as setforth in detail below.

The foregoing dual-modality embodiment may have particular utility insituations where feature sizes are small (e.g., relative to the size offiducial 199), for example, or where an opaque fiducial 199 mayotherwise obscure feature 129 if the two were imaged simultaneously.Fiducial 199 may be specifically colored, textured, shaded, dimensioned,or otherwise manipulated to support the functionality set forth above.It some embodiments, for example, fiducial 199 may be embodied in orcomprise one or more liquid crystal elements or fluorescent materialsdisposed on or incorporated into plate 190; incident light or appliedelectric or magnetic fields may cause or selectively enable fiducialopacity or transparency. Additionally or alternatively, fiducial 199 maybe etched into a surface a plate 190 in such a manner as to create atexture or other surface or structural characteristic enablingsubstantial transparency under certain circumstances and opacity underother circumstances.

FIG. 3 is simplified diagram illustrating geometric aspects of ametrology system constructed and operative in accordance with theembodiment of FIGS. 1A and 1B. The exemplary FIG. 3 geometry may be usedto calculate 3D feature locations from the mathematical relationshipbetween apparent feature-fiducial distances. The two cameras or imagingdevices 111, 112 are simply represented by their imaging planes,identified as Image Plane 1 and Image Plane 2, respectively. As setforth above, these image planes may comprise or be characterized by CCDor CMOS image sensors, for example, or some other image sensortechnology.

A 3D coordinate system, N, may be defined by a set of mutuallyperpendicular unit vectors x, y, and z, where the y axis is normal tothe plane of FIG. 3. The imaging plane of the first imaging device 111(Image Plane 1) may be selectively rotated by an angle, θ₁, about the yaxis, and the imaging plane of the second imaging device 112 (ImagePlane 2) may be selectively rotated by an angle, θ₂, about the y axis.Fiducial 199 and the “tip” of feature 129 may be normally projected ontoeach respective imaging plane, producing respective apparent distancesd₁ and d₂.

Given the apparent distances d₁ and d₂, the position of the feature'stip relative to the fiducial may be described by the distances Δx, Δy,and Δp, where “p” represents “planarity,” or distance in the zdirection. Since both imaging planes are rotated about the y axis,position components on this vector are generally not affected byrotations θ₁ and θ₂. Consequently, the Ay component of the feature'sposition relative to the fiducial may be directly observed on bothimaging planes. The components Δx and Δp may be calculated via equations(1) and (2) as follows. $\begin{matrix}{{\Delta\quad p} = \frac{{d_{1}\cos\quad\left( \theta_{2} \right)} - {d_{2}{\cos\left( \theta_{1} \right)}}}{\sin\quad\left( {\theta_{1} + \theta_{2}} \right)}} & (1) \\\begin{matrix}{{\Delta\quad x} = {{\left\{ \frac{{d_{1}\cos\quad\left( \theta_{2} \right)} - {d_{2}{\cos\left( \theta_{1} \right)}}}{\sin\quad\left( {\theta_{1} + \theta_{2}} \right)} \right\}\tan\quad\left( \theta_{2} \right)} + \frac{d_{2}}{\cos\quad\left( \theta_{2} \right)}}} \\{= {{\Delta\quad p\quad\tan\quad\left( \theta_{2} \right)} + \frac{d_{2}}{\cos\quad\left( \theta_{2} \right)}}}\end{matrix} & (2)\end{matrix}$

If both angles, θ₁ and θ₂, are equal (i.e., θ₁=θ₂=θ), then equations (1)and (2) simplify as follows: $\begin{matrix}{{\Delta\quad p} = \frac{\left( {d_{1} - d_{2}} \right)}{2\quad\sin\quad\theta}} & (3) \\{{\Delta\quad x} = {\frac{\left( {d_{1} - d_{2}} \right)}{2\quad\cos\quad\theta} + \frac{d_{2}}{\cos\quad\theta}}} & (4)\end{matrix}$

FIG. 4 is simplified diagram illustrating geometric aspects of ametrology system constructed and operative in accordance with theembodiment of FIGS. 2A and 2B. In this instance, the angle θ₁ may bedefined as the angle between the lens centerline and the fiducial whenthe lens is at “Lens Location 1” and the image is at one extreme of thefield of view. Similarly, the angle θ₂ may be defined as the anglebetween the lens centerline and the fiducial when the lens is at “LensLocation 2” and the image is at the other extreme of the field of view.

Generally, calculating the angles depicted in FIG. 4 requires accurateknowledge of the location of the stage (reference numeral 130 in FIG.2B) and consequently, the location of the test article (referencenumeral 120 in FIG. 2B) supported thereon. Requiring knowledge of stagelocation is substantially different than requiring accurate motion ofthe stage, i.e., ascertaining the location of the stage in 3D space issignificantly easier than precisely controlling the motion of the stagethrough that space.

In that regard, an inexpensive but highly accurate fiducial plate 190may facilitate accurate determination of stage location. Accordingly,precise measurements of the lateral offset of the fiducial from the lenscenterline in the x direction (distances x₁ and x₂ in FIG. 4) may beobtained. Knowledge of the approximate distance from the lens to thefiducial (λ in FIG. 4) may also facilitate computation of the feature'sposition in 3D space. Given the foregoing variables, the angles θ₁ andθ₂ may then be calculated as follows. $\begin{matrix}{\theta_{1} = {{\tan^{- 1}\left( \frac{x_{1}}{\lambda} \right)} \approx \frac{x_{1}}{\lambda}}} & (5) \\{\theta_{2} = {{\tan^{- 1}\left( \frac{x_{2}}{\lambda} \right)} \approx {\frac{x_{2}}{\lambda}.}}} & (6)\end{matrix}$

Analysis of the sensitivity of θ₁ and θ₂ to λ in equations (5) and (6)reveals that for small angular values, θ₁ and θ₂ are generallyinsensitive to variations or errors in λ. Given the computed values forangles θ₁ and θ₂ and the apparent feature to fiducial distances (i.e.,d₁ and d₂ as illustrated in FIG. 3) at the two lens locations in FIG. 4,the processing required to determine feature location follows the samepath as outlined above with specific reference to equations (1) and (2).

Returning now to the embodiment illustrated in FIGS. 1A and 1B whereθ₁=θ₂=θ, variations on equations (3) and (4) may enable characterizationof the accuracy of the final position measurement for a given feature.Equations (7) and (8), for example, respectively express the accuracy ofthe planarity, δ(Δp), and alignment, δ(Δx), measurements as functions ofboth: variations or errors in the first and second apparent distances,δd1 and δd2; and variations or perturbations in the image plane angle,δθ. $\begin{matrix}{{\delta\left( {\Delta\quad p} \right)} = {{\frac{1}{2\quad\sin\quad\theta}\delta\quad d_{1}} - {\frac{1}{2\quad\sin\quad\theta}\delta\quad d_{2}} + {\left\lbrack \frac{\cos\quad\theta\quad\left( {d_{2} - d_{1}} \right)}{2\quad\sin^{2}\theta} \right\rbrack\delta\quad\theta}}} & (7) \\{{\delta\left( {\Delta\quad x} \right)} = {{\frac{1}{2\quad\cos\quad\theta}\delta\quad d_{1}} - {\frac{1}{2\quad\cos\quad\theta}\delta\quad d_{2}} + {\left\lbrack \frac{\sin\quad\theta\quad\left( {d_{1} + d_{2}} \right)}{2\quad\cos^{2}\theta} \right\rbrack\delta\quad\theta}}} & (8)\end{matrix}$

FIG. 5 is a graph representing data for planarity and alignmentmeasurement sensitivities as functions of imaging device viewing angle,θ. As illustrated in FIG. 5, data representing planarity and alignmentsensitivities (with respect to both variations in apparent distancemeasurement as well as variations about nominal camera viewing angle)were generated for imaging device viewing angles of between about 1° andabout 45°. The data were generated assuming a nominal apparent distanced₁=10 μm and a planarity distance Δp=20 μm.

As is apparent from examination of the FIG. 5 data, for small, shallowviewing angles, planarity accuracy is greatly affected by errors in theapparent distance measurement as well as by variations or perturbationsin the camera angle. At a viewing angle of 2°, for example, every unitof apparent distance error translates into roughly 5 times as great avalue for the planarity measurement error. Alignment accuracy, however,is only nominally affected by apparent distance or camera angle errors;in the case of alignment accuracy sensitivity, each unit of apparentdistance error translates into only about ½ that amount of planaritymeasurement error.

With increasing viewing angles, planarity error generally improves andalignment error generally worsens; as indicated in the FIG. 5 graph, therate at which planarity error improves, however, greatly exceeds therate at which alignment error worsens.

Given a desired accuracy for planarity and alignment measurements, aknown accuracy for apparent distance and camera angle measurements, thedata illustrated in FIG. 5, and the relationships set forth in equations(7) and (8), a corresponding desired or optimal image plane angle 0 maybe calculated for the embodiment illustrated and described in detailabove with reference to FIGS. 1A and 1B.

Equations (7) and (8) show that uncertainty in the image plane angle θmay lead to position measurement error; accordingly, determining theimage plane angle θ with certainty may facilitate minimization orelimination of such errors. Two approaches may be effective indetermining θ with sufficient precision to achieve this goal: (1)precisely construct system 100 and any apparatus supporting imagingdevices 111, 112 to such tolerances that the image plane angle θ isknown to be within the tolerances established by the manufacturingprocess; or (2) allow for looser manufacturing tolerances and measurethe image plane angle 0 to exacting tolerances.

In accordance with some embodiments, for example, image plane angles θ₁and θ₂ for imaging devices 111 and 112, respectively, may be preciselycomputed using a very flat fiducial plate 190 containing at least twofiducial marks having a known spacing. Given the apparent distancebetween the fiducial marks in the two camera views (d_(f1) and d_(f2))and the true distance between the fiducial marks (Δx_(fiducial)), theimage plane angles θ₁ and θ₂ relative to fiducial plate 190 may easilybe computed in accordance with equations (9) and (10). $\begin{matrix}{\theta_{1} = {\cos^{- 1}\left( \frac{d_{f_{1}}}{\Delta\quad x_{fiducial}} \right)}} & (9) \\{\theta_{2} = {\cos^{- 1}\left( \frac{d_{f_{2}}}{\Delta\quad x_{fiducial}} \right)}} & (10)\end{matrix}$

The accuracy of these measured angles is related to the apparentdistance measurement accuracy via $\begin{matrix}{{\delta\quad\theta_{1}} = {- {\frac{\delta\quad d_{f_{1}}}{\Delta\quad{x_{fiducial} \cdot \sin}\quad\theta_{1}}.}}} & (11)\end{matrix}$where θ₂ may be substituted for θ₁ and d_(f2) may be substituted ford_(f1). Due to the sin θ₁ term in the denominator of equation (11), theaccuracy of the θ₁ (or θ₂) measurement degrades with small viewingangles. As is readily apparent from examination of equation (11),measuring the distance between the two most widely separated fiducialsavailable in a single field of view (i.e., employing the greatestavailable value for Δx_(fiducial)) may maximize the accuracy of the θ₁and θ₂ measurements. Additional accuracy may be afforded by averagingmultiple measurements over any other widely spaced fiducials availablein a particular field of view.

As with conventional metrology techniques, lighting may influence theoverall operation of the system and method described herein. In thatregard, it is noted that lighting techniques may affect the ability ofan imaging device to extract detail (such as surface contours ortexture, for example) from an imaged feature; further, lighting effectsmay also influence the geometric appearance of an imaged object orfeature.

Accordingly, functionality and operational characteristics of one ormore illumination sources, the imaging optics employed at imagingdevices 110-112, or both, for example, may be selected eitherindividually or in combination to produce a substantially uniformlyilluminated image of the probe tip or other feature of interest on aparticular test article. In that regard, it will be appreciated that anillumination source or system as well as an optical system comprisingimaging devices 110 or 111, 112 may each have a respective numericalaperture (NA) value. As NA values decrease, for example, the resultingimages reveal more surface feature detail which may complicate the imageprocessing task of ascertaining location through distance measurements.From this standpoint, various components of system 100 (such as theillumination source and devices 110-112, for example) may be constructedor selected such that the illumination system NA and the imaging oroptical system NA are both large.

As set forth in detail above, the exemplary embodiments acquire imagedata from at least two different perspectives. In some applications, thelighting in each perspective may facilitate or allow preservation of thegeometric properties of the object or feature being imaged. For example,if one image perspective employs lighting techniques tending to shift afeature's centroid to the left (the negative x direction, for instance)while the other perspective employs lighting techniques tending to shifta feature's centroid to the right (the positive x direction), theresulting impact on apparent distances to fiducials may produceerroneous location data.

One method of mitigating or eliminating the effects of a shift in imagecentroid due to perspective may comprise preserving the consistency ofimage geometry through implementation of a single, constant illuminationsource, i.e., generally “constant” in intensity and orientation. Forexample, in probe card testing applications designed to identify thelocation of the lowest point on the tip of a probe, the light from anillumination source may be oriented substantially vertically, i.e.,parallel to the z axis in FIGS. 1B and 2B. In such a situation, byLambert's principle, the horizontal surfaces that are characteristic ofthe lowest point on a probe may generate the greatest diffuse reflectionintensity, and allow for correct identification of the lowest point on aprobe. Probe tips, however, also produce specular reflections, which mayreduce the effectiveness of this illumination strategy and produceapparent image shifts due to viewing angle.

Another approach to mitigating the effects of a perspective-inducedshift in image centroid may comprise measuring the shift and accountingfor it. For example, in a probe card analysis application, probelocation measurements may be taken with and without overtravel; themeasurements taken at the overtravel position generally correspond tothe situation where the probes are in contact with the fiducials. Inthis overtravel situation, variations in probe to fiducial distance (d₁and d₂) due to viewing angle should be minimal or eliminated entirely.Any measurable variation, therefore, may be attributed to the effect ofa change in perceived geometry due to viewing angle. Any detected shiftmeasured at the overtravel position may be used to correct measurementstaken without overtravel.

In that regard, the probe to fiducial distances without overtravel (zeroovertravel) may be represented as d_(1ZOT) and d_(2ZOT); similarly, theprobe to fiducial distances with overtravel may be represented asd_(1OT) and d_(2OT).

At overtravel, an average of the two apparent probe to fiducialdistances may generally produce a better estimate of the actualdistance. Given the foregoing, the estimated error in feature geometrydue to changing perspective may be computed by the following:$\begin{matrix}{\hat{e} = {\frac{d_{2_{OT}} - d_{1_{OT}}}{2}.}} & (12)\end{matrix}$

An estimate of probe to fiducial distance in situations withoutovertravel may then be improved using the foregoing estimate.{circumflex over (d)} ₁ _(ZOT) =d ₁ _(ZOT) +ê _(d)   (13){circumflex over (d)} ₂ _(ZOT) =d ₂ _(ZOT) −ê _(d).   (14)

Substituting these perspective-corrected estimations of probe tofiducial distance for d₁ and d₂ in equations (1) and (2), may mitigatethe effects of perspective-induced geometry variation on planarity andalignment measurements.

Several factors may impact the performance of the disclosed embodiments,including, for example: feature centroid identification accuracy;apparent distance measurement accuracy; fiducial location accuracy;optical depth of field (DOF); optical telecentricity; and the NA of theillumination system and imaging optics.

With respect to feature centroid identification accuracy, it is notedthat accurately and repeatably identifying the centroid of a desiredfeature may influence results, as noted above. The surface geometry andoptical reflective properties of the feature may vary considerably fromone test specimen or article to another, and even within a givenspecimen. Acquired images of these features may be processed in such away that the desired area may be correctly identified. In the case of aprobe image for probe card analysis applications, for example, thedesired area is typically the lowest point on the probe. Some methodswhich may have utility in identifying desired areas include connectivity(blob) analysis and pattern recognition.

With respect to apparent distance measurement accuracy, it is noted thatthe accuracy of feature location measurement is generally dependent uponthe underlying accuracy of the device used to acquire images as setforth above with reference to equations (7) and (8) and FIG. 5. Ifimages are captured using a CCD camera, for example, image quantizationand noise may play an important role in the accuracy of apparentdistance measurements. To maximize performance of such embodiments,image quantization size may be made as small (i.e., small pixels) as theparticular application allows; additionally, it may be desirable tomaximize image signal to noise. In that regard, filtering techniques maybe implemented to mitigate the effects of noise. Pixel averaging orinterpolation techniques may be employed to achieve sub-pixel accuracy.

With respect to fiducial location accuracy, it is noted that theexemplary embodiments generally measure the three positional components(i.e., coordinates in 3D space) of a feature of interest relative to aknown fiducial location. Accordingly, inaccuracies in fiducial locationmay translate directly into inaccuracies in feature location. Inapplications employing a plurality or an array of fiducials, featuredistance calculations with respect to multiple fiducials may beaveraged, reducing reliance upon the location accuracy of a singlefiducial.

With respect to optical DOF, it is noted that the disclosed embodimentsimage both a feature and a fiducial simultaneously; these elements aregenerally at different planarity or z distances from the image plane orlens, i.e., the distance from the image plane to the feature will likelybe different than the distance from the image plane to the fiducial.Accordingly, one or both of the feature and the fiducial may not be inoptimum focus. In particular, when viewing a fiducial and a feature thatare laterally separated, the lateral separation combined with theviewing angle produce a depth variation between the objects. In someembodiments, therefore, the optical system (including imaging devices110 or 111, 112) employed to image the feature and the fiducial may beselected to have an adequate DOF to provide sufficient object locationmeasurement accuracy in accordance with system requirements. It will beappreciated that attaining an appropriate DOF may require reducing theNA of the optics, which in turn reduces image resolution. As in anyoptical system, the competing goals of achieving a desired or selectedDOF and maintaining a desired resolution may be balanced according tooverall system requirements. In some implementations, the ScheimpflugRule, for instance, or other optical concepts may be employed tominimize the required DOF.

With respect to optical telecentricity, it is noted that the fiducialand the feature are likely at different distances from the image plane,as noted above; it is generally desirable, however, that this depthseparation not have a significant effect on image magnification. Sincethe measurement of apparent distance between a fiducial and a featuremay generally be affected or influenced by magnification, any opticalmagnification may also affect the resulting position measurement of thefeature. Normally, optical magnification varies with distance from anobjective lens; the degree of such distance-dependence may be customizedor selectively adjusted, for example. Telecentric optics, for instance,exhibit only a bounded variation in magnification as a function ofdistance, Accordingly, some embodiments may employ telecentric optics toacquire image data; the degree of telecentricity may be dictated bydesired measurement accuracy and other system parameters.

As set forth above, aspects of the disclosed embodiments may beimplemented in any general purpose 3D optical metrology application. Byway of example, FIG. 6 is a simplified diagrammatical side view of oneembodiment of a probe card analyzer system constructed and operative inaccordance with the embodiment of FIGS. 1A and 1B. It will beappreciated that the embodiment of FIGS. 2A and 2B may also beappropriate for such a probe card analyzer.

As noted above, fiducial array configurations may comprise one or morefiducials suspended in or etched on a flat, substantially transparentfiducial plate fabricated of glass, quartz, or sapphire, for example. Asuitable imaging array for use in conjunction with the FIG. 6 embodimentmay be a roughly square area array CCD chip; alternatively, the arraymay be rectangular or other configurations generally known in the art.

FIG. 7 is a simplified flow diagram illustrating the general operationof one embodiment of a stereoscopic three-dimensional metrology method.The FIG. 7 method generally represents some or all of the functionalityset forth in detail above; the exemplary operations may be executed orperformed, in whole or in part, by various combinations of the systemcomponents illustrated and described above with reference to FIGS. 1-6.

As indicated at block 701, an article to be imaged (such as asemiconductor wafer, chip, or die, for instance) may be supported,attached, or otherwise disposed on a fixed or movable stage; therelative orientation of an imaging device and the stage, in general, andthe article, in particular, may be selectively controlled or adjustedfor optimal imaging. In that regard, the exemplary embodiments aresusceptible of various modifications in which a selected relativeorientation may be achieved through movement of any or all of thearticle, the stage, or the imaging device. The operation depicted atblock 701 generally represents any of the positioning and orientationstructures and techniques set forth above, irrespective of which systemcomponent is moved to achieve the relative orientation of the articleand the imaging device.

A fiducial plate bearing a fiducial may be interposed between thearticle to be imaged and the imaging device as indicated at block 702.As set forth above, the fiducial plate may be substantially transparent,enabling the imaging device to image a feature on the articletherethrough. In some embodiments, the fiducial plate may bear aplurality of fiducial markings, which may be arranged eitherarbitrarily, for instance, or in a predetermined or ordered array.

Image data may be acquired as indicated at block 703. In that regard,the substantial transparency of the fiducial plate generally allows orenables the imaging device to image both a feature of the test articleand a fiducial on the fiducial plate simultaneously. In embodimentsincorporating a fiducial plate bearing a plurality of fiducial markings,for example, the imaging device may image the feature of the testarticle and selected ones of the fiducial markings which are within thefield of view of the optical system.

A method of measuring the location of the feature may generally measurean apparent distance between the feature and the fiducial from theacquired image data (block 704). While specific geometric aspects of theexemplary embodiments are set forth in detail above with particularreference to FIGS. 3 and 4, various other measurement techniques may besuitable for the operation depicted at block 704 and are accordinglycontemplated herein. Apparent distance measurements may be influenced bysystem component geometry, for example, or other factors such asdistance-dependent magnification effects, lighting or illuminationstrategies, error estimations and correction algorithms, and so forth.Where a fiducial plate bears a plurality of fiducials, the measuringoperation at block 704 may comprise computing an apparent distancebetween the feature and selected ones of the plurality of fiducialmarkings.

As set forth in detail above, a system and method of stereoscopic 3Dmetrology may employ parallactic differences in apparent distancesmeasured in images of the feature and one or more fiducials acquiredfrom multiple perspectives. Accordingly, a determination may be made(decision block 705) whether additional perspective views are desired orrequired.

Where an additional image from a different perspective is desired asdetermined at decision block 705, one or more system components may bemoved or reoriented relative to each other, or perspectives may beswitched (“toggled”) as indicated at block 706. In particular, thearticle, the stage, the imaging device, or some combination thereof maybe moved such that the image plane of the imaging device may be movedfrom a first position to a second position relative to the article(i.e., changing perspectives). Additionally or alternatively, the imageplane may be selectively rotated about one or more coordinate axes asset forth above. In the embodiment illustrated and described above withreference to FIGS. 1A-B and 3, for example, image perspective may betoggled between Image Plane 1 (imaging device 111) and Image Plane 2(imaging device 112), each of which may be selectively oriented at aparticular angle relative to the article to be imaged; additionally oralternatively, the stage and one or both of the imaging devices in sucha two camera arrangement may be translated along a selected coordinateaxis to shift perspectives as indicate at block 706. Subsequent imagedata may be obtained from the new perspective as control loops back toblock 703.

Where an additional image from a different perspective is not desired asdetermined at decision block 705, the feature's position may be measuredor computed as indicated at block 799. Such measurement or calculationmay accurately locate a position of the feature relative to a specificfiducial or a fiducial array, the location of which may be known.Accordingly, accurate identification of the feature's position in 3Dspace relative to the fiducial enables accurate location of the featurerelative to the article and other features thereof.

The FIG. 7 embodiment is presented for illustrative purposes only, andis not intended to imply an order of operations to the exclusion ofother possibilities. By way of specific example, the operations depictedat blocks 701 and 702 may be reversed in order, for instance, orcombined to occur substantially simultaneously; similarly, theoperations depicted at blocks 703 and 704 may be executed concomitantlywhere data acquisition rate and image processing speed are sufficient.Additionally, it will be appreciated that embodiments employing multipleimaging devices may acquire multiple images from different perspectivessimultaneously; parallel paths representing such simultaneous imagingoperations have been omitted from FIG. 7 for clarity. Those of skill inthe art will appreciate that the particular sequence in which theoperations depicted in FIG. 7 are conducted may be influenced by, amongother factors, the functionality and structural configuration of aparticular imaging device or image processing component.

FIG. 8 is a simplified flow diagram illustrating the general operationof one embodiment of an image data acquisition operation. Specifically,the operations depicted in FIG. 8 are directed to an image acquisitionoperation employing multiple imaging modalities. In is noted that theFIG. 8 embodiment generally represents the operations occurring at block703 in FIG. 7.

A first imaging modality (e.g., color, wavelength, intensity ofillumination or other factors) may be selected and set as indicated atblock 801. A feature may be imaged from a particular perspective usingthe first modality as indicated at block 802. As set forth in detailabove, the modality selected at block 801 may be configured to render afiducial transparent during the imaging at block 802.

The imaging technique may be altered to employ a second modality asindicated at block 803. One or more fiducials may then be imaged atblock 804; imaging operations at blocks 802 and 804 may be executed fromthe same perspective in 3D space. As indicated at block 804, the featuremay also be imaged in the second modality. As an alternative to theorder of progression depicted in FIG. 8, it will be appreciated that theoperations executed at blocks 803 and 804 may precede acquisition of animage of the feature using the first modality at blocks 801 and 802.

Image data acquired during the two imaging operations at blocks 802 and804 may be combined to provide a single composite image frame at block805. Accordingly, an apparent distance between the fiducial and thefeature may be obtained (such as by the operation depicted at block 703in FIG. 7) as described above.

The present invention has been illustrated and described in detail withreference to particular embodiments by way of example only, and not byway of limitation. Those of skill in the art will appreciate thatvarious modifications to the exemplary embodiments are within the scopeand contemplation of the present disclosure. Accordingly, it is intendedthat the present invention be limited only by the scope of the appendedclaims.

1. A method of measuring a location of a physical feature on a test article; said method comprising: supporting a test article to be imaged; interposing a fiducial plate bearing a fiducial between said test article and an imaging device; imaging a feature of said test article and said fiducial; and measuring a location of said feature relative to said fiducial.
 2. The method of claim 1 wherein said supporting comprises utilizing a stage movable along any of three coordinate axes.
 3. The method of claim 2 wherein said stage is rotatable about any of said three axes.
 4. The method of claim 3 wherein said imaging comprises selectively orienting said stage relative to said imaging device.
 5. The method of claim 1 wherein said imaging comprises selectively orienting said imaging device relative to said stage.
 6. The method of claim 1 wherein said imaging further comprises selectively translating an image plane of said imaging device relative to said test article.
 7. The method of claim 6 wherein said selectively translating comprises moving said imaging device relative to said test article.
 8. The method of claim 6 wherein said selectively translating comprises moving said test article relative to said imaging device.
 9. The method of claim 1 wherein said imaging comprises acquiring first image data from a first perspective and second image data from a second perspective.
 10. The method of claim 9 wherein said acquiring comprises obtaining said first image data on a first image plane oriented at a first angle relative to said article and obtaining said second image data on a second image plane oriented at a second angle relative to said article.
 11. The method of claim 10 wherein said first angle and said second angle are equal.
 12. The method of claim 9 wherein said acquiring comprises: obtaining said first image data when said imaging device is at a first location relative to said article; selectively adjusting the relative positions of said imaging device and said article; and obtaining said second image data when said imaging device is at a second location relative to said article.
 13. The method of claim 1 wherein said measuring comprises computing an apparent distance between said feature and said fiducial.
 14. The method of claim 13 further comprising selectively repeating said imaging and said measuring.
 15. The method of claim 11 wherein said fiducial plate bears a plurality of fiducials and wherein said measuring comprises computing apparent distances between said feature and selected ones of said plurality of fiducials.
 16. A metrology system comprising: a stage operative to support an article to be imaged; an imaging device selectively oriented relative to said stage and operative to acquire image data on an image plane; and a fiducial plate, bearing a fiducial, interposed between said article to be imaged and said imaging device.
 17. The system of claim 16 wherein said imaging device is operative to image said fiducial and a feature on said article.
 18. The system of claim 17 further comprising an image processing component operative to compute an apparent distance between said feature and said fiducial.
 19. The system of claim 16 wherein at least one of said stage and said imaging device is movable along any of three coordinate axes.
 20. The system of claim 19 wherein at least one of said stage and said imaging device is rotatable about any of said three axes.
 21. The system of claim 20 further comprising a control element operative selectively to control relative movement of said stage and said imaging device.
 22. The system of claim 16 wherein said imaging device comprises a charge-coupled device image sensor.
 23. The system of claim 16 wherein said imaging device comprises a complementary metal oxide semiconductor image sensor.
 24. The system of claim 18 wherein said fiducial plate bears a plurality of fiducial markings.
 25. The system of claim 24 wherein said image processing component is operative to compute apparent distances between said feature and selected ones of said plurality of fiducial markings.
 26. The system of claim 16 wherein said imaging device acquires first image data from a first perspective relative to said article and second image data from a second perspective relative to said article.
 27. The system of claim 26 wherein said first image data are obtained when said image plane is oriented at a first angle relative to said article and said second image data are obtained when said image plane is oriented at a second angle relative to said article.
 28. The system of claim 27 wherein said first angle and said second angle are equal.
 29. The system of claim 26 wherein: said first image data are acquired when said imaging device is at a first location relative to said article; the relative positions of said imaging device and said article are selectively adjusted; and said second image data are acquired when said imaging device is at a second location relative to said article.
 30. The system of claim 16 wherein said article comprises a semiconductor wafer. 